This invention relates to electroplating of plastics, and more particularly to a decorative chrome electroplate on plastic that is free of copper electroplate.
Conventional processes for providing a decorative chrome layer on a plastic substrate generally involve preplating the plastic substrate using an electroless nickel or an electroless copper deposition technique to provide electroconductivity on the surface of the plastic substrate, electrodepositing a layer of copper, electrodepositing one or more layers of nickel over the copper layer, and electrodeposting a layer of chromium over the nickel electroplate. It has generally been believed by those skilled in the art that an electrodeposited layer of copper is required to achieve a high degree of leveling needed for a bright chromium plating. Leveling is defined as the ability of a plating solution to deposit an electroplate having smoother surfaces than that of the preplated plastic surfaces. Substrates having high topographical features require a greater degree of leveling than surfaces with few topographical features. It is also generally believed that the copper layer, which is relatively ductile, is needed to meet thermal cycling requirements, i.e., to facilitate thermal expansion and contraction without deterioration, cracking, flaking or delamination of the composite electroplate from the surface of the substrate. The nickel layer, which is much more noble (corrosion resistant) and tarnish resistant than the copper is needed to provide corrosion protection of the underlying copper layer. The precise composition, thickness and process details for the various layers is dependent on the service environment of the plated product. For example, an exterior automotive part, such as a front end grille or a wheel cover, will generally have thicker layers and will be formulated to withstand a more aggressive environment than a decorative part for a household appliance.
The prevailing belief that a copper sublayer is necessary or desirable is evident from industry standards. Industry standards for several types and grades of electrodeposited copper-nickel-chromium coatings on plastic substrates for applications where both appearance and durability of the coating are important have been established in ASTM B-604-75. This standard specifies the minimum thickness for the copper layer that is needed to meet thermal cycling requirements for various service environments. It is also generally believed that it is necessary to maintain a ratio of copper layer thickness to nickel layer thickness of at least 1:1 in order to achieve successful thermal cycle performance. It has also been believed that when relatively thick nickel and/or chromium layers are used, the ratio of copper layer thickness to nickel layer thickness should be increased to about 2:1. In addition to the ASTM standard, the automotive industry has set minimum electroplate composition and thickness requirements for electroplated plastics. For superior corrosion protection, duplex nickel deposits are used over a copper electroplate. The duplex nickel deposits retard corrosion penetration to the underlying copper electroplate by using a sulfur-free, semi-bright nickel plate under the bright nickel electroplate. When corrosion occurs at a discontinuity in the chromium plate and penetrates through the bright nickel layer to the semi-bright nickel, a corrosion cell allows the more active bright nickel layer to corrode laterally rather than allowing penetration through the semi-bright nickel to the copper layer.
Is has been generally recognized in the industry that it would be desirable to eliminate the underlying copper layer in order to achieve a better appearance when corrosion occurs, because copper forms an undesirable green corrosion product when exposed to marine or industrial atmospheres. It will also be recognized by those skilled in the art that eliminating the copper layer would also have the advantage of reducing the number of process steps involved in preparing a decorative chrome plated article, and could potentially lower product cost. Also, recyclability of finished parts and/or plating waste could be improved if the copper layer is eliminated.
U.S. Pat. No. 3,868,229, entitled xe2x80x9cDecorative Electroplates For Plastics,xe2x80x9d discloses a process for electroplating plastic with a decorative nickel chrome using essentially an all nickel composition by depositing a sublayer of low strength nickel onto a plastic surface which has been made conductive, depositing over the sublayer a super leveling nickel layer followed by deposition of a chromium layer. In order to pass thermal cycle testing, it is disclosed that the ratio of the thickness of the nickel sublayer to the thickness of the super leveling nickel must be at least 2, and the total nickel plate thickness is from about 0.9 to about 1.6 mils. Thus, a disadvantage with the process described by U.S. Pat. No. 3,868,229 is that while it reduces the number of steps required, the total thickness of the nickel layers is significantly greater than the total thickness of the nickel layers in a conventional chromium plating for plastic substrates that has an underlying copper layer. For example, the total thickness of the bright nickel and semi-bright nickel layers that are needed to meet the corrosion and thermal cycle performance requirements of ASTM-604 is typically less than 0.9 mils, whereas the total thickness of the super leveling bright nickel and the non-leveling nickel layers in accordance with the teachings of U.S. Pat. No. 3,868,229 must be from about 0.9 to about 1.6 mils to meet the same requirements. Therefore, any savings associated with elimination of the underlying copper layer is at least partially offset by the added cost associated with using thicker nickel layers.
In view of the above discussion, it is evident that there remains a need for a process for depositing a decorative electroplate on plastics which does not include an underlying copper electroplate layer, and which meets corrosion and thermal cycle test requirements without requiring thicker nickel layers.
In one aspect, the invention provides a process for depositing a decorative chrome electroplate on a plastic substrate without requiring a copper electroplate sublayer, while utilizing very thin nickel electroplate layers. The process reduces the number of steps required for forming a decorative chrome electroplate on a plastic substrate, and reduces the number of electroplate baths needed, without requiring additional nickel, thereby reducing the cost of a finished product.
The process of this invention generally comprises steps of electrodepositing on an electrically conductive coating a high leveling semi-bright nickel electroplate layer, electroplating on the high leveling semi-bright nickel electroplate layer a bright nickel electroplate layer, and electrodepositing over the bright nickel electroplate a layer of chromium.
The decorative chromium plating prepared in accordance with the process of this invention is capable of passing corrosion and thermal cycle test requirements without an electrodeposited copper layer, while having a total thickness of nickel layers that is about equal to or less than the total thickness of conventional chrome platings exhibiting the desired corrosion resistance and thermal cycling characteristics.
These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.